1. Field of the Invention
The present invention relates to a turbo-supercharger for an internal combustion engine, and more particularly to a turbo-supercharger having an exhaust turbine provided with a plurality of exhaust gas inlet passages and valve means for closing one or more of the exhaust gas inlet passages so that the effective exhaust gas inlet passage area can be changed in accordance with the engine operating conditions.
2. Description of the Prior Art
Conventional turbo-superchargers for internal combustion engines include on exhaust gas turbine having a turbine casing provided with a single exhaust gas inlet passage leading to a turbine chamber where the turbine rotor is located. Since the turbine efficiency is governed by an A/R ratio wherein A is the area of the exhaust gas passage in the turbine casing and R is the radius of the turbine casing scroll, the conventional design has been such that the A/R ratio is determined so that the highest turbine efficiency can be accomplished under a medium speed engine operation. It has been considered that, with this design, a relatively high efficiency can be obtained throughout the engine operating range.
It should however be noted that, under the conventional design wherein the A/R ratio is determined so that the best result is obtained in the medium speed engine operating range, the quantity of the exhaust gas is beyond an optimum value under a high speed range so that there will be an increase in the flow resistance and a consequent increase in the back pressure at the engine exhaust ports. The increase in the flow resistance disturbs an effective utilization of the energy in the exhaust gas so that the turbine efficiency is decreased. The increase in the back pressure causes a poor combustion efficiency so that a sufficient engine output cannot be obtained.
Further, under a low speed engine operation, the quantity of the exhaust gas is lower than an optimum value so that the exhaust gas flow is decreased. As the result, the exhaust gas cannot drive the turbine rotor with a sufficiently high speed so that the turbine efficiency is undesirably decreased.
In order to overcome the disadvantages of the conventional turbo-supercharger, it has been proposed to separate the exhaust gas inlet passage in the turbine casing into a plurality of passage sections and close one or more of the passage sections under a low speed engine operation by means of an appropriately provided valve mechanism. For example, in the Japanese patent application No. 56-116426 filed on July 27, 1981 and published for public inspection on Feb. 3, 1983 under the disclosure number 58-18522, there is disclosed a turbo-supercharger including a turbine casing in which the turbine scroll defining the exhaust gas inlet passage is formed with a radial partition wall which divides the exhaust gas inlet passage into two parallel inlet passage sections. The first one of the inlet passage sections is opened at the inlet end of the turbine casing so as to be connected with the exhaust passage of the engine. The second inlet passage section has an inlet end connected with the first passage section through a communication port formed in the partition wall to open to an intermediate portion of the first passage section. The turbine casing carries a valve which closes the communication port under a low speed engine operation so that the exhaust gas is passed only through the first passage section to the turbine chamber where the turbine rotor is located.
The proposed turbo-supercharger is considered as being advantageous in that the effective area of the exhaust gas inlet passage can be changed in accordance with the engine speed so that a turbine efficiency can be obtained under a wide engine speed range. It should however be noted that the proposed arrangement has disadvantages due to the fact that the communication port is formed in the partition wall separating the two passage sections. One of such disadvantages is that, under a high speed operating range, a part of the exhaust gas flows into the second passage section from the inlet part of the one passage section through the communication port abruptly changing its direction of flow so that the flow resistance is increased under a high speed range. Further, under a high speed engine operating range, the exhaust gas flow through the communication port has a tendency of flowing in a direction inclined with respect to the axis of the communication port so that the effective area of the port is decreased and the flow resistance is increased. The problem may be solved by increasing the diameter of the communication port. However, this solution causes an increase in the dimensions of the valve mechanism.
Further disadvantages are derived in the proposed arrangement due to the fact that the valve mechanism is provided on the turbine casing. At first, it should be pointed out that complicated thermal deformations are produced in the turbine casing so that it is difficult to maintain a satisfactory seal between the valve member and the valve seat which is formed in the turbine casing. It is further difficult to provide a space for locating the valve mechanism in the turbine casing.
In the copending patent application Ser. No. 681,829 filed on Dec. 14, 1984, now U.S. Pat. No. 4,617,799, and assigned to the same assignee of the present invention, there is disclosed a turbo-supercharger having a turbine casing which is formed with a substantially planar inlet end surface where two mutually separated exhaust gas inlet passages are opened. This application further teaches to use the inlet end surfaces as the valve seat for the valve mechanism and to carry the valve member not on the turbine casing but on the engine exhaust pipe. The application does not however teach in detail how the valve member should be located in its open position.